The spine is constituted by superposed vertebrae that are normally in alignment along a vertical axis, going from the lumbar vertebrae to the cervical vertebrae, each vertebra presenting a posterior wall from which there projects a spinous process and two sides having walls from which there project the ribs and/or transverse processes. When the spine of an individual presents abnormal curvature, the vertebrae are inclined relative to one another and relative to said vertebral axis. The sides of the vertebrae situated on one side are thus moved closer together forming a concave side, whereas the sides of the vertebrae on the other side are spaced apart from one another and form a convex side.
In order to straighten the spinal column, the sides of the vertebrae on the concave side are spaced apart from one another and moved relative to one another to distances that are substantially equivalent to those between the sides of the vertebrae on the other side. In order to keep the vertebrae in that relative positioning, known devices have screws that are inserted in the vertebrae or hooks that are inserted along the inside wall of the vertebral canal, and rods that are for interconnecting the screws or the hooks.
The hooks are generally inserted in pairs into each vertebra and on either side close to the pedicles, with their heads projecting from the posterior wall of the vertebra, one on either side of the spinous process. By way of example, the heads form a socket suitable for receiving a rod that is held in place by means of a nut screwed onto the head so as to press against the rod. The rows constituted by the heads of the hooks situated on either side of the spinous processes are interconnected and held in a fixed position by two rods that are parallel to each other and to the axis of the spine.